1. Field of the Invention
The present invention relates to light scattering type smoke detectors, and in particular to an optical arrangement for such a detector which allows a relatively short structural length of the detector.
2. Description of the Prior Art
Smoke detectors operating according to the light scattering principle are known in the art, such as is disclosed, for example, in German OS 26 30 843, corresponding to U.S. Pat. No. 4,103,997. Detectors of this type have an optical transmitter which is directed toward a test chamber, an optical receiver disposed at an acute angle .beta. with respect to the transmitter, and is also directed toward the test chamber in such a manner that the receiver receives back-scattered radiation scattered by the particles present in the test chamber. Known devices also utilize optical diaphragms for guiding the radiation for the transmitter and receiver. The principle of operation of such smoke detectors is based on the scattering of electromagnetic radiation incident on smoke particles present in the test chamber. For a uniform detection of large and small smoke particles, it is preferable to utilize only the back-scattered light, that is, that light which is scattered by the particles at an acute angle .beta. to the optical transmitter.
Known structures of this type, however, have the disadvantage that the structural length of the device is large in relation to the transmission and reception angles which are utilized. This is because the radiation is guided exclusively by means of the diaphragms. In devices of this type, a zone in the general shape of a cravat is formed by the overlapping of the fields of view of the transmitter and receiver, and it is this zone which determines the minimum required structural length of the arrangement in accordance with the following equation: ##EQU1## wherein h is the structural length, b is the distance of the transmitter from the receiver, .beta. is the angle between the respective optical axes of the transmitter and the receiver, and .alpha. is the angle of the fields of view of the transmitter and receiver.
As can be seen by this equation, the area of the overlap zone is maximized when the angle .beta. is made as small as possible and the angle .alpha. is made as large as possible. This results, however, in a large structural length h. In order to achieve a short structural length h, at a given angle .beta., it would be necessary to reduce the angle .alpha..
This is illustrated in FIG. 1 which schematically represents the detection zone in conventional smoke detectors of the type described above. An optical transmitter 1 has a field of view angle .alpha., and a receiver 2 has a substantially equal field of view angle .alpha.. The transmitter 1 and the receiver 2 are separated by a distance b. The optical axes of the transmitter 1 and the receiver 2 form an angle .beta. with respect to each other. The fields of view of the transmitter 1 and receiver 2 overlap in an overlap zone OZ which is generally in the shape of a cravat. This zone represents the test chamber or detection zone in which smoke particles to be identified are present. The tip of the overlap zone faces away from the transmitter 1 and receiver 2 and results in the structural length h.